A nobleman had a sitting-room with only one
window in it - a square window, 1 meter high and 1 meter wide - as is
shown in the diagram. He had weak eyes, and the window gave too much
light. He sent for the builder, and told him to alter the window, so
as to give half the light. Only he was to keep it square - the same 1
meter high and 1 meter wide. How did he do it? The builder wasn't
allowed to use curtains, or shutters, or colored glass, or anything of
that sort.

*This is a retelling of the problem posed by
Lewis Carroll in a letter to Helen Fielden, then fourteen, which he
wrote in March 1873.